1. Field of the Invention
The present invention relates to an electronic flash device of an automatic light adjusting type. More particularly, the present invention relates to an electronic flash device of an automatic light adjusting type in which flash light can be automatically adjusted without an error in the adjustment.
2. Description Related to the Prior Art
A lens-fitted photo film unit is known, in which a housing is pre-loaded with unexposed photo film and which includes a taking lens, a shutter device and other relevant elements. There is a flash type of lens-fitted photo film unit in which a flash device is incorporated for applying flash light to a photographic object to be taken typically at night or indoors. The flash device in the available type of the lens-fitted photo film unit applies flash light at a predetermined amount irrespective of an object distance. In a close-up photography, there arises a problem of an overexposure of a principal object due to a very near distance. The principal object is likely to be photographed in a whitish manner as viewed in a print produced from the frame. Even if the principal object is reproduced with suitable density, a background image is likely to be too dark.
An auto flash device is known as a flash device of an automatic flash emitting type. In the auto flash device, a flash discharge tube emits flash light. The photographic field reflects the flash light, which is received by a photoreceptor element. A photoelectric current is obtained by the photoreceptor element, and integrated. When the result of the integration reaches a predetermined level, a non-contact switch such as a thyristor is changed over, to discontinue the discharge of the main capacitor. The flash emission of the flash discharge tube is quenched. Thus, the light amount of the flash light is optimized.
JP-B 52-047327 (corresponding to U.S. Pat. Nos. 3,783,336, 3,809,951, 3,818,266, 3,857,064, 3,992,643 and 4,164,686) discloses a construction for preventing errors in operation of a light adjusting circuit due to influence of unrelated light, for example, light from other flash Devices. In FIG. 10, the flash device includes a main capacitor 80, a light adjusting circuit 81 and a power source circuit 83. A capacitor 82 in the light adjusting circuit 81 is charged by the power source circuit 83 together with the main capacitor 80. When a trigger voltage is applied by a trigger circuit 84 to a flash discharge tube 85, the main capacitor 80 is discharged in a path with the flash discharge tube 85, which emits flash light.
At the same time as the start of flash emission, the capacitor 82 starts being discharged in a path with the flash discharge tube 85, a Zener diode 86 and a resistor 87. Voltage generated across the Zener diode 86 is applied to the light adjusting circuit 81 which starts operation. A photo diode 88 receives and reflected flash light from the object. An integration capacitor 89 stores charge according to the light amount of the reflected flash light at the photo diode 88. When the charged voltage across the integration capacitor 89 reaches a predetermined level, a UJT (unijunction transistor) 90 is turned on. A turn-on capacitor 91 is responsively discharged in a path with a resistor 92. Voltage generated across the resistor 92 is applied to a gate of a thyristor 93. The thyristor 93 is turned on, to discharge the main capacitor 80 in a path with the thyristor 93. The flash emission is quenched.
Should another flash device happen to exist beside the camera, the light adjusting circuit 81 is likely to receive influence of the external flash device, to turn on the thyristor 93. The main capacitor 80 may be discharged accidentally without any purpose. However, the construction according to the prior art can prevent such a problem, because the light adjusting circuit 81 operates at the same time as the start of flash emission.
However, there are several problems in the construction in the above document. Failure is likely to occur in the automatic light adjustment specifically when the object distance is a very near distance, or when a photographic object has a very high reflectance. Flash light is emitted in full emission without restriction, to result in an overexposure. The turn-on capacitor 91, which starts being charged simultaneously with the flash emission, has the voltage coming up to a level enough for turning on the thyristor 93. The failure occurs when the UJT 90 is turned on by an increase of the integration capacitor 89 before the voltage across the turn-on capacitor 91 comes up to the level enough for turning on the thyristor 93. This is because a light amount of reflected flash light is remarkably high.
A photo transistor is used as photoreceptor element. When the voltage is applied to the photo transistor as effective voltage for light detection, the photo transistor outputs a current in amplification of a current input to the photo transistor because of junction capacitance. The integration capacitor 89 is charged by the current after the amplification. A problem arises, as it is likely that the flash emission is quenched earlier than it should be. The precision in the light adjustment is lower.
In view of the foregoing problems, an object of the present invention is to provide an electronic flash device of an automatic light adjusting type in which flash light can be automatically adjusted without an error in the adjustment.
In order to achieve the above and other objects and advantages of this invention, an electronic flash device of an automatic light adjusting type includes a main capacitor for storing charge at a high voltage, a flash discharge tube for generating flash light upon discharging the main capacitor therewith, and a light adjusting circuit for adjusting a light amount of the flash light by feedback. In the flash device, the light adjusting circuit includes a reflected light measuring unit for outputting an integration voltage for representing a light amount of reflected light from a photographic field illuminated with the flash light. A non-contact switch short-circuits terminals of the main capacitor upon being turned on, to quench emission of the flash light in the flash discharge tube. A turn-on capacitor is charged by a current of discharge in response to the emission of the flash light, and is discharged when the integration voltage comes up to a prescribed level, to turn on the non-contact switch responsively. A control switching element keeps the reflected light measuring unit disabled while the turn-on capacitor is initially charged, and enables the reflected light measuring unit after voltage across the turn-on capacitor comes up to a predetermined level.
Consequently, full emission of flash light can be prevented specifically for photography at a very near distance.
Furthermore, a discharge switching unit is connected in parallel with the turn-on capacitor, for being turned on when the integration voltage from the reflected light measuring unit comes up to the prescribed level, to discharge the turn-on capacitor.
The reflected light measuring unit includes a photo transistor, supplied with electric power by the control switching element, for outputting a photoelectric current proportional to intensity of the reflected light incident thereon. There is an integration capacitor for integration of the photoelectric current to output the integration voltage.
Furthermore, a bypass switching element is turned on in response to becoming conductive of the control switching element, and short-circuits terminals of the integration capacitor until lapse of a predetermined time, to inhibit charging with the photoelectric current.
Consequently, flash light can be automatically adjusted without an error in the adjustment.
The bypass switching element is a bypass transistor, having a collector and emitter connected with respectively electrodes of the integration capacitor, and a base connected with the control switching element. Furthermore, a capacitor is connected between the base of the bypass transistor and the control switching element, and adapted for determining the predetermined time at which on-voltage is applied to the base of the bypass transistor.
The reflected light measuring unit includes a near distance compensation resistor, connected in series with the integration capacitor, for constituting a serial circuit, to correct the integration voltage if the integration voltage increases slowly and highly, the serial circuit generating the integration voltage at a corrected level.
Furthermore, an effective voltage generator outputs increasing power source voltage when the main capacitor is discharged. The turn-on capacitor is supplied with the power source voltage, and charged.
The effective voltage generator includes a power supply capacitor, connected in parallel with the main capacitor and the flash discharge tube, for being charged and discharged together with the main capacitor, the power supply capacitor outputting an increasing power supply voltage upon being discharged. A Zener diode is connected with the power supply capacitor, for limiting the power supply voltage from the power supply capacitor to at least a predetermined effective voltage. When the increasing power supply voltage comes up to the predetermined effective voltage, the control switching element is rendered conductive in response to an output of the Zener diode, for applying the power supply voltage to the reflected light measuring unit.
The non-contact switch is a thyristor having an anode and cathode connected with respectively electrodes of the flash discharge tube, and having a gate connected with the turn-on capacitor.
Furthermore, a current limiting resistor is connected between the power supply capacitor and the Zener diode, for limiting a current from the power supply capacitor.
In a preferred embodiment, the power supply capacitor is connected in parallel with the main capacitor and the flash discharge tube at a connection point Pd. The Zener diode is connected in parallel with the turn-on capacitor. Furthermore, a choke coil is connected between the main capacitor and the connection point Pd, for constituting a first serial circuit with the main capacitor, wherein a current flows in the choke coil during the flash emission of the flash discharge tube by discharging the main capacitor, there occurs back electromotive force in the choke coil, for lowering a voltage between terminals of the first serial circuit, and for lowering a voltage between terminals of a second serial circuit including the power supply capacitor and the Zener diode, to discharge the power supply capacitor to charge the turn-on capacitor.
In another preferred embodiment, furthermore, a level changer sets the prescribed level variable for the integration voltage, to determine the light amount of the flash light to be emitted before quench.
The level changer has an offset voltage generator, including a first resistor and a variable resistor connected in series therewith, for generating an offset voltage by dividing the power supply voltage according to resistance of the variable resistor. An adjustment transistor is supplied with input voltage, for becoming conductive when the input voltage comes up to a predetermined voltage, the input voltage being lower than the integration voltage by the offset voltage.
The adjustment transistor has a collector and emitter one of which is supplied with the offset voltage. Furthermore, an effective voltage generator applies the power supply voltage to the light adjusting circuit. A capacitor is connected with the one of the collector and emitter in parallel with the effective voltage generator and the first resistor, for absorbing electric noise to prevent the adjustment transistor from erroneously becoming conductive.